ESTIMATING TOUCHDOWN POINT FROM FOQA DATA
Here are some notes on the way we estimate Touchdown point for our FOQA system for the B777. This is one method. The most interesting is to use a FOQA track overlay on a surveyed and scaled satellite photo of the airport and note where the MLG goes from Tilt to No-Tilt. We only do this for hard landings mostly as they are mostly short landings. I am not sure why someone cannot automate a GPS touchdown point and overlay it onto a known lat/long series for a particular RW off the Jepp database.
• Find a point on the approach where there is surveyed flat terrain (e.g. at 100 ft RA for a CAT II RW).
• Add the correction between Radio Alt and MLG (approx 21.2 ft). This figure is derived from the assumption (airline specified) that when aft MLG touchdown happens at say 5° Pitch the RA should read 0 ft. Your airline technical department should know this.
• Apply correction for RA ht due to terrain at the Cat 11 DH which has been surveyed usually. This gives the height of the NLG Glide Slope aerial at that point.
• Then correct for G/S deviation (1 dot = 0.35 degrees) and for the actual TCH. If TCH is 50 ft then the G/S antenna is 954 ft down the RW etc. Some simple geometry. Therefore you have one side and one angle of a triangle and can calculate height of the gldeslope at that point and hence the Distance To Go.
• Since RA is measured each second and the aircraft is travelling at xxx fps it will travel xxxx ft before touchdown. You should determine the distance from the glide slope receiver on the NLG doors to the aft MLG wheel and that will tell you at your specified FOQA derived Radio Altitude that the aft MLG is XXXX ft from the EOR and has XXX ft to run so touchdown will be XXX ft from the end of the RW.
• To be most accurate the effect of changing ground speed (if any) between that point and touchdown must be calculated. And to be very accurate add in the effect of aircraft pitch, MLG tilt and the MLG strut extension.
All very pedantic and time consuming but worth it for the valuable training lessons that come from FOQA data. This is a very condensed verion of the process but I hope it helps. It is only going to be accurate to within say plus or minus 125 ft but thats not too bad to pick up the major causal factors in hard landings.